This invention relates to high temperature alloys, and more particularly to intermetallic compounds comprising ruthenium and aluminum, herein referred to as ruthenium aluminides, having high hardness at elevated temperatures and good room temperature toughness.
Intermetallic compounds are alloys having a simple stoichiometric proportion between the components and having a crystal structure different from the crystal structure of the component elements. The structure of intermetallic compounds is homogeneous over a typically narrow composition range where atoms of each component occupy ordered sites in the crystal lattice. Many intermetallic compounds have been studied because of their potential for use at elevated temperatures. The compounds can have greater stiffness than the metals from which they are formed, and have higher strength at elevated temperatures as compared to disordered alloys. In many cases low specific gravities give intermetallic compounds a high ratio of stiffness-to-density and strength-to-density, two quantities that are highly desirable in aircraft or rotating parts.
A serious problem in the use of intermetallic compounds comes from their tendency toward brittleness. Brittleness in intermetallic compounds is shown by poor ductility or poor toughness at low temperatures such as room temperature. Toughness is the ability of a material to absorb impact energy. A result of such brittleness is that many intermetallic compounds cannot be formed extensively and the articles that can be formed are susceptible to damage in their normal use and handling.
A well known intermetallic compound system is the titanium aluminides. Many of the advances from the research of titanium aluminides produced alloys having a reduced tendency toward brittleness while maintaing a high strength at elevated temperatures. For example in U.S. Pat. No. 4,292,077 to Blackburn et al., trititanium aluminides consisting of about 24-27 atomic percent aluminum, 11-16 atomic percent niobium, and the balance titanium are disclosed as having good high temperature strength with low temperature ductility. The Blackburn alloys are disclosed as being useful at temperatures of about 600.degree. C.
It is well known within the metallurgical art that indentation hardness is an indicator of the yield strength of materials, "The Indentation of Materials by Wedges," Hirst, W., Howse, M. G. J. W., Proceedings of the Royal Society A., Vol. 311, pp. 429-444 (1969). Therefore a comparative determination of the high temperature strength of different materials can be made from comparing the high temperature indentation hardness of the materials.
An additional concern for high temperature materials is the resistance of the material to oxidation. For example, titanium aluminide intermetallic compounds are considered to have good oxidation resistance up to about 600.degree. C. because of the formation of stable aluminum oxide scales on the surface of such alloys.
An object of this invention is to provide intermetallic compounds having a high hardness and high strength at temperatures up to about 1150.degree. C., and good toughness at room temperature.
Another object is to provide intermetallic compounds having good oxidation resistance up to about 1000.degree. C.